1. Field of the Invention
The present invention relates generally to magnetic fluid seals. Particularly, the present invention relates to multi-stage magnetic fluid seals.
2. Description of the Prior Art
Magnetic fluid rotary seals have been widely used in vacuum applications over the past twenty years. The basic structure of the seal has at least one magnet, a rotary shaft, and pole pieces fastened within a housing. The magnet, the pole pieces and the shaft form a magnetic circuit with air gaps. A magnetic fluid is attracted to the air gap and forms the dynamic sealing between the pole pieces and the rotary shaft. The sealing between stationary parts such as between a pole and the housing is usually accomplished by using a rubber O-ring at the radial interface. Modern applications increasingly require magnetic fluid seals with increased pressure capacities. Conversely, as the size of modern applications decreases, smaller magnetic fluid seals having the same pressure capacity are also needed.
The pressure capacity of a magnetic seal is proportional to the magnetic field within the seal. When the magnetic field is concentrated, or increased, the pressure capacity of the seal also increases proportionally.
Protrusions or ridges, which are also referred to as stages, projections, teeth, or fins, have been incorporated within the gap between a pole piece and a shaft of a magnetic fluid seal to concentrate the magnetic field adjacent the pole piece. These ridges can be formed in the shaft, in the pole, or in both the shaft and the pole. As the number of ridges or teeth increases, the pressure capacity of the seal also increases. However, the sustained differential pressure for each stage is proportional to the total flux of the magnetic field even if two pole pieces are used, one on each side of the magnet. Thus, such a magnetic system has an upper limit and saturation develops at a relatively small number of teeth or ridges. At magnetic saturation, an increase in the number of teeth will reduce the flux choking and will better utilize the magnetic flux.
In situations where magnetic saturation does not exist such as when the magnet is not strong enough or when the pole pieces are increased in size to the limit of the total flux of the magnetic field of the existing magnet, further increases in the number of ridges by increasing the size of the pole piece will result in lesser and lesser increases in pressure capacity. This is so because the magnetic flux field beneath each additional ridge is not strong and centrifugal forces easily throw the magnetic fluid away from the gap.
To further increase the sustained differential pressure, a seal requires multiple magnets and pole pieces. However, it is not always practical to simply increase the size of the magnetic seal. Attempts have been made to increase the sustained pressure capacity for each stage seal below a pole piece of a magnetic seal thereby increasing the pressure capacity of the magnetic seal without increasing the size of the magnetic seal.
U.S. Pat. No. 3,3620,584 (Rosensweig, 1971) discloses several embodiments of a magnetic fluid seal with knife edges cut into the outer ring pole pieces or the shaft, or both. A plurality of knife edges form a row of right triangles where the acute angles of the plurality of knife edges are aligned in one direction. In another embodiment viewed in cross-section, the acute angles of the knife edges are grouped into two groups. The first group of knife edges has the acute angles aligned in one direction and the second group of knife edges has the acute angles aligned in the opposite direction.
U.S. Pat. No. 4,440,402 (Pinkus, 1984) discloses a ferrofin magnetic-fluid seal. The ferrofin magnetic-fluid seal comprises a plurality of concentric, fin-like projections of magnetically permeable material formed on each of a rotating member and a stationary member in a spaced-apart opposing relation defining a plurality of magnetic gap regions. The cross-sectional shape of the fin-like protrusions are rectangular in geometry with parallel sides and a parallel base and top. The dimensions of the fin-like projections are such that the distance between the base and top is greater than the distance between the sides. The fin base is attached to the shaft and the pole pieces. The fin top protrudes into the gap between the shaft and pole pieces.
Magnetic Fluids, Engineering Applications (Berkovsky et al., 1993, p. 138–41) discloses that the pressure differential can be increased slightly when tapered teeth (serving as focusing structures of the magnetic field) are located on both the poles and the shaft, one opposite another. The cross-sectional view of the tapered teeth disclosed in Berkovsky et al. form an equilateral triangle where each leg of the triangle is the same length. Berkovsky further discloses that a seal with tapered teeth is disadvantageous since the structure must be fixed in both radial and axial directions. Additionally, Berkovsky discloses that, since working gaps are small (about 0.2 millimeters), problems arise with serviceability of shafts and high shaft runout.
The eccentric location of the shaft and the poles due to high shaft runout causes changes in the working gap in the azimuthal direction, which causes magnetic field intensity changes in the gap between the shaft and the poles. The point at which the gap has increased has a correspondingly decreased magnetic field strength and, thus, a decreased holding capacity of the seal. This decrease may be appreciable. The reduction in sealing capacity due to eccentricity can be more than 80–90%, depending on the level of eccentricity.
U.S. Pat. No. 5,954,342 (Mikhalev, 1999) discloses a magnetic fluid seal apparatus for a rotary shaft. The magnetic shaft sleeve of the apparatus includes a plurality of protrusions affixed thereto. The protrusions are triangularly shaped having an acute angle. The acute angle provides a frictional bond with the magnetic fluid. One group of sleeve protrusions that aligns with one pole has acute angles lined up facing in one direction. The second group of sleeve protrusions that aligns with the second pole has acute angles lined up facing the opposite direction.
Even though the prior art knife edge stages help focus the magnetic flux lines in the air gap and thus slightly increase the differential pressure capacity, they also increase the magnetic choking effect with regard to the density of flux lines at the knife edges, which is limiting. Where double, opposed knife edges are used, misalignment causes a decrease in the magnetic force field.
Therefore, what is needed is a multistage magnetic fluid seal that provides a higher pressure capacity than conventional magnetic fluid seals of similar size. What is also needed is a multistage magnetic fluid seal that focuses the magnetic force field and provides a decreased choking effect. What is further needed is a multistage magnetic fluid seal having stages on both opposed surfaces of the rotary seal that is much less sensitive to axial misalignment than conventional multistage seals having stages on both opposed surfaces of the rotary seal.